1. Field of the Invention
The present invention relates generally to power supplies, and more particularly, to current sharing and equalization techniques among multiple DC-to-DC and AC-to-DC power modules.
2. Description of Related Art
It is often advantageous to implement a power system using a plurality of individual DC-to-DC or AC-to-DC power supplies connected in parallel. The DC power supplies may be stand-alone power supplies or may be power modules designed for integration into larger power supplies or power storage. (xe2x80x9cPower supplyxe2x80x9d in this context conventionally refers to a voltage/current converter, not to the ultimate source of electric current such as a battery or generator). Unlike a single module power supply, the multi-module power system can provide for failure recovery if one module ceases to operate. Furthermore, simply supplementing the design with additional power supplies or power modules may increase the total current capacity of a multi-module power system. Often such power systems are used in telecommunications equipment and other equipment requiring a reliable source of power, e.g., matrix switches and industrial controllers.
Following Kirchhoff""s voltage law, the total current delivered to a load from a power system having multiple power modules configured in parallel equals the sum of the currents delivered by each individual power module. In other words, the current supplied by each power module contributes to the total load current supplied by the power system. If one module delivers a greater amount of current, that module will also dissipate more power and therefore become hotter than the other power modules. Higher operating temperature normally yields reduced reliability of the overall power system. Therefore, there is a goal of evenly distributing the task of generating the total load current among parallel-connected power supplies or power modules.
FIGS. 1A and 1B illustrate two different power system configurations, each using multiple power supplies. FIG. 1A illustrates a power system 10 having multiple power modules 100, 101, 102, 103 configured in parallel supplying power to a load 40. Each module accepts an input voltage VDD 20 and provides an output current I0, I1, I2, I3 to a power system output node 30. The sum of the individual module output currents is supplied to load 40. The total load current ILOAD=I0+I1+I2+I3 results in a voltage VLOAD across the load referenced between output node 30 of power system 10 and a ground 50. Without some form of feedback control, power system 10 will be unable to control and equalize the currents I0, I1, I2, I3 supplied by respective modules 100, 101, 102, 103.
If the current supplied by the power system is evenly divided among the power modules, each power module will deliver an equal amount of power. By evenly dividing the task of providing power among the power modules, no one power module will be driven to an extreme that may cause power conversion inefficiencies, power module degradation or premature power module failure. To evenly distribute the power load among the plurality of power modules, an external controller may be used to sense and adjust each module""s current output. Alternatively, the power modules may be designed to communicate among each other and self regulate their output power. For example, a power system may be designed such that each module communicates its current output to other power modules and each module adjusts its output based on the received signal.
Some power systems utilize a single wire or twisted pair configured as a shared bus to communicate the maximum current supplied by any one of the parallel-connected power modules. In these configurations, each of a plurality of power modules is connected to a shared bus. Each power module attempts to raise the voltage on the shared bus to a value indicative of the current supplied by that power module. The power module providing the greatest current to the load overrides the voltage provided by the other power modules. The voltage level on the shared bus therefore corresponds to a level indicating the current supplied by the power module providing the most current.
FIG. 1B illustrates a power system having such a current-share bus. The input node 20 and output node 30 of the power system are equivalent to those previously described with reference to FIG. 1A. Unlike FIG. 1A, each module 100, 101, 102, 103 in the power system 10 of FIG. 1B communicates with the other modules by way of a current-share bus 200. The current-share bus 200 may be a single wire providing a signal relative to a common ground of the power system 10.
As well as providing a voltage indicative of a power module""s output current level, each power module also monitors the shared bus to determine the maximum current supplied by any one of the power modules. If each power module is providing the same amount of current to the load, the voltage applied to the current-share bus set by each module is equal to the voltage monitored by each module from the shared bus. Any power module providing a level of current below that which is indicated on the current-share bus will detect that at least one module is providing more current and thus more power than it is providing. A module providing less current than that indicated on the shared bus will incrementally increase its output voltage, which in turn will increase the current supplied to the load, until the current supplied by the power module equals the current indicated on the current-share bus. In this way, each of a plurality of parallel power modules will increase its output current in an attempt to track the output current supplied by the module providing the most current.
Each power module also monitors the output voltage supplied by the multi-module power system. As some power modules increase their current outputs, the total output voltage of the power system provided to the load may exceed the voltage demanded by the load. Each power module providing a current equal to the current indicated on the current-share bus will reduce its output current until the voltage provided to a load by the power system equals the desired voltage. With time, the power modules work in tandem to evenly distribute the current supplied by the power modules and to provide a regulated output voltage to the load. If the load""s power demands change over time, the power modules track the changing demand by adjusting the current supplied by each module. If current sharing is operating properly, the resulting steady-state output currents I0, I1, I2, I3 of each respective module 100, 101, 102, 103 will be approximately equal to each another.
FIGS. 2A and 2B show examples of power modules that include circuitry allowing the modules to communicate via a shared bus. FIG. 2A shows a power module 100A that interfaces to a single-wire current-share bus that carries a shared analog signal representing an averaged signal. A plurality of power modules connected in parallel, such as the one shown in FIG. 2A, result in a voltage level on the current-share bus 200 that represents the average current of all of the modules.
Power module 100A includes a power regulator 110 and feedback circuitry including a current sensor 120, a current-to-voltage converter 130, interface circuitry 140 to the current-share bus 200, a voltage error amplifier 150A, and interface circuitry 160 to the power regulator 110. Power regulator 110 generates an output current IOUTPUT. Power regulator 110 may be one of any of a number of power converter types, including for example, buck, boost, buck-boost or other current-providing power module well known in the art. Feedback circuitry in the power module, separate from any feedback circuitry within power regulator 110, provides a feedback voltage VFEEDBACK to power regulator 110. Power regulator 110 contains its own feedback circuitry (not shown) to control the output voltage of the power regulator. The feedback voltage VFEEDBACK alters the internal feedback circuitry of the power regulator 110 to provide current sharing, as will be further described below.
Current sensor 120 monitors the output of the power regulator 110 and provides a signal to current-to-voltage converter 130 indicative of the output current IOUTPUT. Current-to-voltage converter 130 translates the signal indicative of the output current to an analog voltage level. This voltage level is coupled to one input of voltage error amplifier 150A. The voltage level is also passed through a resister 140, which is connected to current-share bus 200. Resistor 140 in combination with similarly situated resistors of other power modules (not shown) average the voltage levels supplied by each power module. The averaged voltage on current-share bus 200 represents the average current supplied by all of the power modules connected to current-share bus 200. The voltage residing on current-share bus 200 is supplied as a second input to voltage error amplifier 150A.
The voltage error amplifier 150A determines the difference between the output voltage of converter 130 and the average voltage level provided by current-share bus 200. If the difference is positive, the output current IOUTPUT is greater than the average current of the power modules. To equalize the output currents of each power module, voltage error amplifier 150A and resistor 160 generate a feedback voltage VFEEDBACK that directs power regulator 110 to adjust the output current. Power regulator 110 uses this feedback voltage VFEEDBACK to decrease the output voltage of the regulator.
Alternatively, if the difference between the input voltages is negative, the output current IOUTPUT is less than the average current of the power modules. To equalize the output current provided by each module, voltage error amplifier 150A will increase the feedback voltage VFEEDBACK provided through resistor 160. In response, power regulator 110 increases the output voltage, which in turn increases the output current IOUTPUT. One drawback to this design is that if current-share bus 200 shorts to ground, each power module will drive its output voltage towards zero volts.
FIG. 2B shows another power module 100B that interfaces to a single-wire current-share bus that also carries a shared analog signal. A plurality of parallel-connected power modules connected to a common current-share bus 200, such as the power-module 100B shown in FIG. 2B, results in a voltage level on current-share bus 200 that represents the maximum current provided by any one of the power modules. The design of power module 100B functions substantially as described above with reference to power module 100A in FIG. 2A except, for example, the interface to current-share bus 200 and associated circuitry is modified. The voltage level provided by current-to-voltage converter 130 is passed through diode 170, which pulls up current-share bus 200 to at least the output voltage level of converter 130, assuming the voltage drop across diode 170 is negligible. If any one of the other power modules pulls current-share bus 200 to a value higher than the voltage level provided by converter 130 of module 100B, diode 170 will be reversed biased and current-share bus 200 will be unaffected by this power module. As a result, current-share bus 200 is held to the highest value produced by the power module generating the greatest output current.
Error amplifier 150B has two input signals: (1) a negative input providing a voltage level offset by VOFFSET; and (2) a positive input providing the maximum voltage level sent to current-share bus 200 by all of the power modules. The first input signal is equal to the output voltage level of converter 130 increased by an offset voltage VOFFSET. The offset in voltage helps to stabilize the feedback loop by helping to set a clear master, i.e., a power module that produces slightly more current than the other modules. If the resulting offset voltage level at the negative input is greater than the maximum voltage level riding on current-share bus 200, voltage error amplifier 150B provides a lower feedback signal VFEEDBACK. In this case, diode 190 prevents passing of this feedback signal to power regulator 110 and the output voltage of regulator 110 remains unchanged. Alternatively, if the resulting voltage level at the negative input is less than the maximum voltage level on current-share bus 200, voltage error amplifier 150B provides a higher feedback signal VFEEDBACK through the serially connected diode 190 and resistor 160 thereby increasing the output voltage and in turn the output current IOUTPUT of power regulator 110.
Such known systems have additional drawbacks. First, a system using an analog shared bus communicating an amplitude signal is susceptible to line noise on the bus. Noise can be generated by sources within the power system itself or can be generated by energy radiating from the load or neighboring electronic circuitry. Noise on the current-share bus may erroneously drive the power modules to inaccurate and unpredictable output currents. Second, each power module might have a slightly different ground reference point. If a first power module has a lower ground reference than another power module, a voltage provided to the shared bus by the second power module will appear to the first power module as representing a larger current than actually exists. Third, parasitic resistances in the power module circuitry may reduce the actual voltage supplied to the current-share bus. Thus, the voltage supplied to the current-share bus by a power module may not accurately indicate the supplied output current by a power module.
Thus, it would be desirable to provide a current sharing and equalization technique for use with multiple DC-to-DC and AC-to-DC power modules that overcomes these and other disadvantages of the prior art.
According to embodiments of the present invention, methods and apparatus are provided for current sharing and equalization among a plurality of power modules configured in a parallel arrangement in a power system.
More particularly, a method of sharing current among a plurality of power modules is provided. The method includes sensing of a characteristic of an output power signal of at least one of the plurality of power modules and providing a first signal having a pulse width corresponding to the sensed characteristic. The first signal is imparted onto a current share bus coupled to each of the plurality of power modules if the first signal has a pulse width greater than corresponding first signals of other power modules coupled to the current share bus, whereupon one of the first signals from the plurality of power modules having greatest pulse width is imparted onto the current share bus as a second signal. A phase difference between the first signal and the second signal is detected and a feedback signal is provided to the at least one power module in response to the detected phase difference. The feedback signal thereby controls the at least one power module to regulate the output power signal.
In another embodiment, a power module is provided for operation with a plurality of like power modules connected together to provide a common output. The power module includes a power regulator providing an output power signal on a corresponding output terminal, and a bus interface adapted to communicate with a current share bus that is connected in like manner to each of the other power modules. The power module further includes a feedback loop adapted to sense the current level of the output power signal and provide a feedback signal to the power regulator in response thereto. The feedback signal thereby controls the power regulator to regulate the output power signal. The feedback loop further includes a converter adapted to provide a first signal having a pulse width corresponding to the sensed current level, and an error controller adapted to detect a phase difference between the first signal and a second signal received through the bus interface from the current share bus. The feedback loop imparts the first signal onto the current share bus if the first signal has a pulse width greater than corresponding first signals of the other power modules communicating with the current share bus, whereupon the first signal becomes the second signal.
A more complete understanding of a current share method and system will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings, which will first be described briefly.